Cladding and Assembly Technology of Medium-Pressure Steam Superheater Tube Box Study Note

Literature Overview and Background

Medium-pressure steam superheaters are critical components in power generation and process heating systems, where they raise the temperature of saturated steam to the desired superheated state. The tube box, which serves as the header connecting the superheater tubes to the steam supply and return lines, is subjected to temperatures ranging from 350 to 550 degrees Celsius and pressures of 3.8 to 6.5 MPa. In applications involving corrosive flue gases or process media, the tube box interior surfaces require corrosion-resistant cladding to ensure long-term integrity.

The literature under review addresses the design, cladding, and assembly of a medium-pressure steam superheater tube box using weld overlay technology to create a corrosion-resistant interior surface. The tube box is fabricated from carbon steel or low-alloy steel for structural strength, with a stainless steel or nickel-based alloy overlay applied to the interior surfaces that are exposed to the corrosive steam and flue gas environment.

Core Technical Content

Tube Box Design and Material Selection

The tube box is a complex pressure-containing component with numerous tube holes, nozzle connections, and internal baffles. The design must satisfy the requirements of GB/T 150 or ASME VIII Div.1 for pressure vessel design, with additional considerations for thermal cycling and corrosion resistance.

The base material selection must consider the mechanical properties at design temperature, the impact toughness requirements (typically 47 J at -20 degrees Celsius per GB/T 150), and the weldability. The overlay material must provide adequate corrosion resistance in the service environment while maintaining good bonding strength to the base material and acceptable thermal expansion compatibility.

Cladding Process for Tube Box Components

The tube box consists of multiple components: the main box plates, tube plates, nozzle extensions, and internal baffles. Each component requires cladding on its interior surface. The literature evaluates the following cladding approaches:

For the main box plates, the cladding process typically involves:

1. Surface preparation by grinding to Ra 12.5 micrometers, removing all mill scale and contaminants.
2. Preheating to 150-250 degrees Celsius depending on the carbon equivalent of the base material.
3. Application of a transition layer using a low-dilution stainless steel wire (such as ER309L) with SAW, using a current of 500-700 amperes and a travel speed of 400-600 mm/min.
4. Application of the final overlay layer using 316L or Inconel 625 wire with SAW, building up to the required thickness of 3-5 mm.
5. Post-weld stress relief at 600-650 degrees Celsius for 2 hours per 25 mm of thickness.

Assembly and Welding Sequence

The assembly of the cladded tube box requires careful planning to prevent distortion and to protect the overlay layer during welding of the assembly joints. The literature recommends the following sequence:

1. Component cladding: All individual components are cladded and stress-relieved before assembly.
2. Fit-up: Components are fitted up with precise alignment, ensuring that cladded surfaces are in contact where required.
3. Assembly welding: Assembly welds are made using a procedure that minimizes heat input into the cladded surfaces. The welding sequence is planned to minimize distortion, typically starting from the center and working outward.
4. Tube hole drilling: After assembly and stress relief, tube holes are drilled and reamed through the cladding layer. The drilling must be performed with care to avoid damaging the overlay layer at the hole edges.
5. Tube installation: Superheater tubes are inserted and welded to the tube holes using a code-approved procedure. The weld procedure must account for the dissimilar metal interface between the tube material and the overlay layer.
6. Final inspection: NDT of all welds, dimensional verification, and hydrostatic testing.

Key Welding Parameters for Assembly

Defect Analysis and Countermeasures

Quality Control Plan

The literature emphasizes a comprehensive quality control plan based on the following:

Engineering Practice Insights

The literature presents a detailed case study of a tube box for a 5.2 MPa, 480 degrees Celsius superheater in a power plant. The tube box was fabricated from 16MnR plates with a 316L overlay on the interior surfaces. The total overlay area was approximately 150 square meters, with a total overlay thickness of 4 mm.

The cladding process was performed using dual-wire SAW with a 309L transition layer (1.5 mm) and a 316L final layer (2.5 mm). The dilution rate was measured at 12 percent, and the overlay hardness was 210 HV, consistent with the expected properties of 316L stainless steel.

A critical challenge encountered during the project was the drilling of 320 tube holes through the 4 mm overlay layer. The literature recommends using a carbide-tipped drill with a feed rate of less than 0.1 mm/rev and a coolant flow of at least 20 liters/min. The drilling was performed in two stages: first, a pilot hole was drilled through the overlay layer using a smaller drill (10 mm diameter), followed by a full-size drill (42 mm diameter) using a step-drilling approach. This method minimized the risk of overlay delamination at the hole edges.

The assembly welding was performed using a qualified procedure that included a 309L root pass (to bridge the dissimilar metal interface) followed by 316L fill and cap passes. The heat input was limited to 25 kJ/mm to prevent excessive thermal distortion of the cladded surfaces. Post-weld stress relief was performed at 620 degrees Celsius for 3 hours.

The hydrostatic test was performed at 7.8 MPa (1.5 times the design pressure) with a 30-minute hold. No leaks were detected, and the dimensional verification confirmed that the tube box geometry was within the specified tolerances (less than 2 mm total distortion).

Key Questions and Reflections

The literature raises important considerations regarding the long-term performance of the overlay layer under thermal cycling conditions. The thermal expansion mismatch between the 16MnR base material (12.5 x 10^-6 /K) and the 316L overlay (16.5 x 10^-6 /K) generates cyclic thermal stresses at the interface. Over thousands of thermal cycles, this can lead to fatigue cracking at the interface. The literature recommends that the design should include a fatigue assessment based on the thermal stress range, and that the overlay thickness should be limited to prevent excessive thermal stress buildup.

Another important consideration is the impact of the tube-to-box weld on the overlay layer. The welding of the superheater tubes to the tube box introduces localized heat input that can affect the overlay layer microstructure and properties in the vicinity of the weld. The literature recommends that the weld procedure be qualified to include the dissimilar metal interface, and that the weld heat input be carefully controlled to minimize the heat-affected zone in the overlay layer.

Summary and Implications

The cladding and assembly of medium-pressure steam superheater tube boxes is a complex engineering challenge that requires careful integration of design, material selection, cladding process, and assembly welding. The key to success lies in the careful control of dilution during cladding, the optimization of the welding sequence to minimize distortion, and the rigorous quality control of all welds. Engineers should pay particular attention to the thermal management during assembly welding, as excessive heat input can degrade the overlay layer properties and compromise the corrosion resistance of the tube box. The economic justification for cladding is strongest in applications where the corrosive environment would otherwise require the use of expensive alloy materials for the entire tube box, or where the service life of an uncladded tube box would be unacceptably short.